ON A METHOD TO DISPROVE GENERALIZED BRUNN–MINKOWSKI INEQUALITIES

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ON A METHOD TO DISPROVE GENERALIZED BRUNN–MINKOWSKI INEQUALITIES NICOLAS JUILLET Abstract. We present a general method to disprove generalized Brunn–Minkowski inequalities. We initially developed this method in [14] in the particular case of the sub-Riemannian Heisenberg group in order to prove that this space does not satisfy a curvature- dimension condition in the sense of Lott–Villani and Sturm. Introduction New developments in analysis and geometry deal with a synthetic definition of Ricci curvature in the non-smooth context of metric spaces, whereas Ricci curvature originated in smooth Riemannian manifolds. Precisely, the property for a space to satisfy a so-called curvature- dimension condition CD(K,N) is interpreted as behaving in some as- pects as a Riemannian manifold with dimension ≤ N and Ricci cur- vature ≥ K at any point. Lott and Villani [18, 17] and independently Sturm [24, 25] managed to define a new notion of curvature-dimension CD(K,N) using optimal transport, a tool that was traditionally used in probability and statistics. They exploited some nice aspects of this theory. Two of them are — (i) the theory can be developed on very gen- eral sets (typically on Polish metric spaces (X, ?)), (ii) the geodesics of the Wasserstein space (a metric space made of the probability measures used in optimal transport) are represented as a probability measure in the space of the geodesics of (X, ?).

Abstract.We present a general method to disprove generalized Brunn–Minkowski inequalities. We initially developed this method in [14] in the particular case of the sub-Riemannian Heisenberg group in order to prove that this space does not satisfy a curvature-dimension condition in the sense of Lott–Villani and Sturm.

Introduction New developments in analysis and geometry deal with a synthetic deﬁnition of Ricci curvature in the non-smooth context of metric spaces, whereas Ricci curvature originated in smooth Riemannian manifolds. Precisely, the property for a space to satisfy a so-called curvature-dimension conditionCD(K, N) is interpreted as behaving in some as-pects as a Riemannian manifold with dimension≤Nand Ricci cur-vature≥Kat any point. Lott and Villani [18, 17] and independently Sturm [24, 25] managed to deﬁne a new notion of curvature-dimension CD(K, N) using optimal transport, a tool that was traditionally used in probability and statistics. They exploited some nice aspects of this theory. Two of them are — (i) the theory can be developed on very gen-eral sets (typically on Polish metric spaces (X, ρ)), (ii) the geodesics of the Wasserstein space (a metric space made of the probability measures used in optimal transport) are represented as a probability measure in the space of the geodesics of (X, ρdetails about geodesics (in the). For sense of minimizing curves) and curves in metric spaces see for instance [1, 5]. Up to now there are two concepts of families of curvature-dimension. The ﬁrst family is connected to the correspondence with the curvature-´ dimension theory of Bakry and Emery [3] and provides results in dif-fusion semi-group theory, such as logarithmic Sobolev inequalities (see ´ [2]). In the eighties, Bakry and Emery introduced the criterionCDBE(K, N)